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Abstract- In this paper, a crossed dipole antenna combined with the first iteration quadratic Koch curve geometry, which has a novel configuration, is proposed ...
MULTIBAND CROSS FRACTAL DIPOLE ANTENNA FOR UHF AND SHF APPLICATIONS

Fawwaz J. Jibrael, Wafaa S. Mummo, Mahmoud T. Yaseen Department of Electrical and Electronic Engineering University of Technology Baghdad, Iraq e-mail: [email protected]

successfully used to model such complex natural objects as galaxies, cloud boundaries, mountain ranges, coastlines, snowflakes, trees, leaves, ferns, and much more. Since the pioneering work of Mandelbrot and others, a wide variety of applications for fractals continue to be found in many branches of science and engineering. One such area is fractal electrodynamics [5-7], in which fractal geometry is combined with electromagnetic theory for the purpose of investigating a new class of radiation, propagation, and scattering problems. One of the most promising areas of fractal-electrodynamics research is in its application to antenna theory and design. Traditional approaches to the analysis and design of antenna systems have their foundation in Euclidean geometry. There has been a considerable amount of recent interest, however, in the possibility of developing new types of antennas that employ fractal rather than Euclidean geometric concepts in their design. We refer to this new and rapidly growing field of research as fractal antenna engineering. Because fractal geometry is an extension of classical geometry, its recent introduction provides engineers with the unprecedented opportunity to explore a virtually limitless number of previously unavailable configurations for possible use in the development of new and innovative antenna designs.

Abstract- In this paper, a crossed dipole antenna combined with the first iteration quadratic Koch curve geometry, which has a novel configuration, is proposed and investigated for low profile

and

multi-band

performance

in

communication

systems. The proposed antenna has operating frequencies of

1255 MHz, 2058 MHz, and 6450 MHz. The field pattern of the proposed antenna is described and simulated in the three planes:

XZ-plane, YZ-plane, and XY -plane.

characteristics,

The radiation

VSWR, reflection coefficients, polarization,

and input impedance of the proposed antenna are described and simulated using the 4NEC2 software package. Also, the gain of this proposed antenna is calculated and described in the three planes: XZ-plane, YZ-plane, and XV-plane, where the antenna is placed in the YZ-plane.

Keywords- multiband antenna, Koch curve, dipole antenna, fractal antenna I.

INTRODUCTION

There has been an ever-growing demand, in both the military as well as the commercial sectors, for antenna designs that possess the following highly desirable attributes: 1. Compact size 2. Low profile 3. Conformal 4. Multi-hand or broadband

II.

There are a variety of approaches that have been developed over the years, which can be utilized to achieve one or more of these design objectives. An excellent overview of various useful techniques for designing compact (i.e., miniature) antennas may is found elsewhere [1,2]. Moreover, a number of approaches for designing multi-band (primarily, dual­ hand) antennas have been summarized in [3]. Recently, the possibility of developing antenna designs that exploit in some way the properties of fractals to achieve these goals, at least in part, has attracted a lot of attention. The term fractal, which means broken or irregular fragments, was originally coined by Mandelbrot [4] to describe a family of complex shapes that possess an inherent self-similarity or self-affinity in their geometrical structure. The original inspiration for the development of fractal geometry came largely from an in-depth study of the patterns of nature. For instance, fractals have been

GENERATION OF KOCH CURVE ANTENNA

The procedure employed in the generation of Koch curve antenna is stated as follows: • starting with a straight line called initiator, as shown in Figure (1), this initiator is portioned or divided into four equal parts, and the length of each part is (1/4) of the initiator length. • This curve is generated by repeatedly replacing each line segment, composed of four quarters, with the generator consisting of eight pieces, each one quarter long (see Fig.l) [8]. Each smaller segment of the curve is an exact replica of the whole curve. There are eight such segments making up the curve, each one represents one-quarter reduction of the original curve. Different from Euclidean geometries, fractal geometries are characterized by their non-integer dimensions. Fractal dimension

978-1-4244-5849-3/10/$26.00 ©2010 IEEE

219

(2a) with first iteration quadratic Koch curve geometry. The proposed antenna is shown in Figure (2b).

contains information used about the self-similarity and the space-filling properties of any fractal structure [9]. The fractal similarity dimension (FD) is defined as [8]:

FD

==

( 10g (1/

10g N ) &

)

==

( log (4) 10g 8)

==

1. 5

Where N is the total number of distinct copies, and

( 1/ E) is

the reduction factor value which means how will the length of the new side be with respect to the original side length.

Iteration ° ltemt;onl

Iteration

� -------­

Initiator

Generator

(a)

2

I I Iteration

3

.�� � � .� �

Figure I. First three iterations of the construction of the quadratic Koch curve

III.

DESIGN OF PROPOSED FRACTAL CROSS DIPOLE ANTENNA

(b) Figure 2. Cross Dipole Antenna (a) cross nonnal dipole antenna (b) proposed cross dipole antenna

The proposed antenna has been simulated using numerical modeling commercial software 4NEC2, which utilizes method of moment based software. The Method of Moment (MoM) is used to calculate the current distribution along the proposed antenna, and hence the radiation characteristics of the antenna [10]. The 4NEC2 program is used in all simulations. This is very effective in analyzing antennas that can be modeled with wire segments, such as the one under consideration here. To suit the requirements, the antenna is modeled in the absence of dielectric, although some of the practical implementations do require dielectric support [11]. The steps of the proposed antenna design are shown below:

Step 2:

The proposed antenna is placed in YZ-plane with design frequency equal to 750 MHz with its feed source point located at the origin (0,0,0), and this source is set to 1 volt, as shown in Figure (2). Step 3:

For design frequency of 750 MHz, the design wavelength Ao is 0.4 m (40 cm), then the length of each arm in the proposed antenna geometry is 10 cm, as shown in Figure (2b).

Step 1:

The proposed antenna geometry includes the replacement of each arm in the normal dipole crossed antenna in Figure

220

TABLE I Resonant Frequencies and Input Impedances for Proposed Antenna

Visualization of this proposed antenna, using NEC-viewer package, is shown in figure (3).

Input impedance (n)

Frequency (MHz)

- ..... � .. - .. ,

1255 2058 6450

2.

Figure 3. Visualization of the Modeled Dipole Antenna Geometry

IV.

X

j3.5498 -512.839 ci15.936

VSWR of the antenna is shown in Figure 5. It is found that the proposed antenna has triple-band behavior at the resonance frequencies 1255 MHz, 2058 MHz, and 6450 MHz with acceptable bandwidth for reflection coefficients < -10 dB, at these frequencies (VSWR< 2). Table II shows these resonant frequencies and the VSWR and reflection coefficients of each one.

SIMULATION RESULTS AND DISCUSSION

In this work, Method of Moment simulation code (NEC) is used to perform a detailed study of VSWR, reflection coefficient, gain, polarization, input impedance and radiation field pattern of the proposed cross dipole antenna in a free space. 1.

R

35.7719 72.5153 77.2978

• .

.



The real and imaginary parts of the input impedance of this proposed antenna are shown in Figure 4 over a frequency range from 0 GHz to 7 GHz. The antenna's input impedance characteristics show the multiple resonance characteristics. Table I shows these resonant frequencies and the corresponding input impedance of each one.

��--�--��--�-�--�--�.---� "'"�DoUOO

Figure 5. Simulated 50n, VSWR vs. frequency

TABLE II VSWR and reflection coefficient of the Proposed Antenna

Frequency (MHz)

VSWR

Reflection coefficient (dB)

Bandwidth (MHz)

1255

1.412

-15.349

52

2058

1.53294

-13.539

160

6450

1.65387

-12.168

672

3.

Figure 4. Antenna input impedance

221

The gain of the proposed antenna is calculated by using 4NEC software in the three planes are XZ­ plane, YZ-plane, and XV-plane, where the antenna is placed in the YZ-plane, as shown in Table III.

The polarization of the antenna is linear in YZ-plane and elliptical in XZ-plane and XY-plane.

TABLE III The Gain of the Proposed Antenna at the Resonant Frequencies in the Three Planes Gain (dB i)

Frequency

4.

(MHz)

XZ-plane (phi=O)

YZ-plane (phi=90)

XY-plane (theta=90)

1255

1.95

3.94

1.26

2058

6.82

1.84

6.82

6450

5.01

1.04

4.25

The radiation patterns at these resonant frequencies in the planes YZ-plane, XZ-plane, and XV-plane are depicted in Figure 6, where the antenna is placed in the YZ-plane.

(b) YZ-plane

(c) XY-plane

(a) XZ-plane

Figure 6. Radiation Patterns of the Modeled Antenna in the three planes (a) XZ-plane (b) YZ-plane (c) XY-plane

222

V.

CONCLUSION

A novel small size and multi-band cross dipole antenna based on a fractal first iteration quadratic Koch curve, has is presented. The analysis of the proposed antenna is performed using the method of moments (MOM), and the numerical simulations show that the proposed antenna has the ability to work as multi-band antenna at the frequencies 1255 MHz, 2058 MHz, and 6450 MHz. In addition, this antenna has VSWR